Bioenergetic studies include the glycolytic and oxidative pathways which convert metabolites into chemical energy and the conversion of this energy for cellular functions. Studies of bioenergetic modifications in neoplastic cells identify alterations in the glycolytic pathway and in ultrastructure, including mitochondria. Lactate dehydrogenase is modified in a chemically-transformed neoplastic cell line from rat liver compared with its non-transformed control, showing a higher overall rate of reaction. The results support the increased synthesis of lactic acid found in these and other neoplastic cells. The change in kinetic behavior is accompanied by modifications in the proportions of LDH-4 and -5, isozymes associated with the liver, and with a marked increase in LDH with an isoelectric point of 8.8. The mitochondria show decreased numbers and pleomorphic structures in the neoplastic cell line. They are displaced in the cytosol, along with other organelles. One energy utilization pathway seems unchanged, the transport of protons. This was determined by a spectrophotometric change in the intracellular dye, 6-carboxyfluorescein, relative to pH. These experiments were carried out with monolayers of anchorage-dependent cells grown on Leighton tube slides and examined by split beam/dual wavelength spectrophotometric techniques. In transformed cells, both lactate and hydrogen ions diffuse from the cytosol into the medium and also are exported to a small extent by active transport processes as in the control cells. These studies show that the pH of the cytosol is more acidic in the transformed cells, for these cells convert ten times as much glucose to lactic acid as the control cells.